YOU might think that the physical bounds of your body are a neat cut-off point where “you” end and the rest of the world begins. Yet everywhere you go you leave behind a calling card. Whether it is hair, skin cells, sweat or waste material such as urine and faeces, you are constantly shedding material that started off as part of you.

How much of ourselves do we unknowingly dump each day on our clothes, on the way to work, in the office, on door handles, cellphones and in the bathroom? And once it has left our body, does it still belong to us?

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A commonly cited figure for hair loss is 50 hairs a day. “Everyone says it, but no one really knows where it originated,” says Henry Lee founder of the Henry C. Lee Institute of Forensic Science in West Haven, Connecticut. How many skin cells and how much sweat we deposit in the wider world is even more of a mystery. As for things such as earwax, spit, snot, faeces and the like, it seems we prefer not to think about them, let alone quantify them.

What we do know is how much of our DNA is cast out in bodily detritus. “Blood and semen contain the most,” says Lee. Every millilitre of semen has up to 300,000 nanograms of DNA. For blood, the figure is 30,000 nanograms, while saliva, another rich source, contains a tenth …